PART I.] Prevalence of Disease in Relation to Condition of Soil. 195 



(5.)— Relations which the different conditions of Soil bear to one another.— 



(Charts II, III, IV.) 



The most important point to be noted in regard to this subject is the apparent 

 dependence of the amount of carbonic acid in the soil on the degree of soil-moisture. 

 When the latter is high, the carbonic acid is at its maximum, and the minimum 

 periods of both also coincide generally. The facts already pointed out in regard to 

 the behaviour of the carbonic acid-contents of the individual layers in reference 

 to the rain-fall and water-level, very clearly indicate such a dependence ; for, whilst 

 the carbonic acid of the upper layer coincides more closely with the rain-fall than 

 with the water-level, the reverse relation appears in the case of the lower layer of 

 soil. 



(6.)— Comparison of the prevalence of Disease with the occurrence of various 

 conditions of Soil in regard to Carbonic Acid, Temperature and Water-level,— 



(Charts II III.) 



As regards the prevalence of cholera on comparing the figures and charts on 

 this point, the only remarkable coincidence appears to lie in the converse relation 

 which water-level, and in a less marked degree rain-fall, bear to the prevalence of 

 this disease. When the latter is at a maximum, the water-level is at a minimum, 

 and when the water-level is at a maximum, the prevalence of cholera is at a minimum. 

 There is no such close coincidence either in regard to conditions of soil-temperature 

 or amount of carbonic acid, although, in so far as soil-moisture appears to determine 

 the amount of carbonic acid in the soil, there is a general coincidence in regard 

 to the latter also. The relations between rain-fall and prevalence of cholera are not so 

 strongly marked as those between the latter and the water-level ; and it even appears 

 as though the converse relation between conditions of water-level and prevalence of 

 cholera were in some degree more distinct than the direct one between the water-level 

 and the rain-fall. 



The greatest prevalence of fevers during the period of observation occurred 

 coincidently with the period of maximum carbonic acid and highest water-level. 



There were two maximum periods of dysentery, one occurring during the rise 

 in the water-level, and the other at a corresponding point in the course of its fall. 

 No coincidence can be traced in regard to the other conditions of soil, save the 

 carbonic acid of the upper layer which in this part of its course very closely 

 corresponds with the water-level. 



No very clear connection can be traced between the statistics of total mortality 

 and the prevalence of any special conditions of soil. There were two periods of 

 maximum mortality during the period of observation — one in November and December, 

 coincident with marked prevalence of fever and dysentery ; the other in April and 

 May with maximum cholera. 



The comparison of the prevalence of disease with the existence of special 



